1. Field of the Invention
The present invention relates to linear actuators and, in particular, to a moving coil linear actuator which utilizes the interaction of magnetic circuits to provide a desired air gap flux density in an actuator of minimal diameter or, alternatively, maximum force within a specified device envelope.
2. Discussion of the Prior Art
Linear actuators are electromagnetic devices that provide linear mechanical motion in response to the interaction of magnetic and electrical circuits.
As shown in FIG. 1, a typical "single-ended" moving coil linear actuator consists of a cylindrical inner core and an outer shell which surrounds the inner core to define an annular space between the two. An annular magnet of a certain polarity is mounted on the inner wall of the shell. A second corresponding annular magnet of opposite polarity is mounted on the outer wall of the core. These two magnets combine to form a magnetic circuit which generates a magnetic flux within the air gap between the two magnets. The air gap receives a moving coil assembly, or armature, which includes a cylindrical coil winding which is connected to an appropriate current source. In accordance with basic electrical principles, when current is supplied to the coil, a magnetic field is generated which opposes the magnetic field generated by the magnetic circuit. The opposing magnetic fields impart a force to the coil assembly which is proportional to the magnetic flux density in the air gap and current in the coil. This force is utilized to cause linear reciprocating movement of the coil assembly.
Linear actuators are utilized in many high precision applications since they suffer less wear and produce less contaminating particles than do conventional crank-shaft piston assemblies. For example, in the simplest case, the linear movement of the armature may be used as a direct substitute for the linear motion provided by a rotary actuator of the type that requires a linkage assembly between a rotary motor and an actuator arm. By modifying the basic linear actuator device to incorporate a hollow core which receives longitudinally mounted sensors, the device may be used as an extremely sensitive position sensor, i.e. a "smart" actuator. By incorporating a piston structure and a fluid port that extends from the piston chamber through the outer wall of the shell, the device may be utilized as a compressor.
However, single-ended linear actuators of the type described above are inherently unbalanced and require the incorporation of additional mass strictly for the purpose of balancing the device. In some applications, this additional balancing mass is incorporated only with a corresponding decrease in generated activator force in order to meet specified device envelope requirements.
To compensate for the unbalanced characteristics of single-ended actuators, "back-to-back", double-ended actuator designs of the type illustrated in FIG. 2 are utilized in some applications. This double-ended design comprises, essentially, two independent single-ended devices of the type described above placed end-to-end. Each of the two independent magnetic circuits activates its respective coil assembly armature to produce self-cancelling strokes and, hence, a balanced device.